In a color copying machine or a color facsimile system using photo-electric cconversion elements such as CCD's, optical mirrors and dichroic mirrors are employed to read a color picture in different wavelength ranges. A plurality of image signals thus read are processed by a color separation circuit, so as to provide desired image signals which have been color-separated. The color picture is processed for every picture element as described above; however, it is not always true that the color-separated color signals exactly represent the colors of the respective picture elements of the color picture. The reasons for this are as follows:
(1) The chromatic aberration of the image forming lens.
(2) In the case where a plurality of reading systems are employed, these reading systems are different from one another in the magnifications of the central and peripheral portions of the lens.
(3) When a plurality of reading systems are used, the optical axes of the reading systems are not in alignment with one another, and the remaining systems differ in the focal length of the lens.
(4) When analog data outputted by the photoelectric conversion element is binary-encoded, the image signals are different in threshold level.
Because of the above-described mechanical, physical or optical reasons, even if excellent optical components are arranged with high accuracy and with ordinary resolution, a ghost on the order of two bits maximum may be formed in the main scanning direction and a ghost on the order of one bit may be formed in the auxiliary scanning direction. The term "ghost" is intended to mean a region of data R (representing red) which, as shown in FIG. 1, is formed on the boundary between the region of data B (representing black) and the region of data W (representing white), on the output side of a reading device adapted to read black and red. In this case, the original color picture is shown in FIG. 2. If the color image data is as shown in FIG. 2, then the color image data has been correctly read and no ghost has been formed.
If the ghost is formed, then not only are images recorded or displayed on a display unit with low quality color image data, but also an image editing device may recognize the ghost as an actual line or line drawing, thus making errors in the editing operation. All the thin lines outputted by the reading device may be eliminated as ghosts. However, if this method is employed, then almost all the effective image data are removed, which provides worse results.